Cancer is a major lethal disease for humans and is caused by physiologically-uncontrolled cell proliferation which affects normal physiological conditions of human body resulting in serious pathological reactions often leading to death. Although tremendous efforts on cancer studies and treatments have been made, presently, cancer is still the major cause of death to humans. There are multiple approaches to treat cancer patients including surgery, radiation therapy, and chemotherapy. As the first two methods are not able to completely eliminate cancer cells in patients, the latter approach is commonly used to control cancer cell growth with or without other treatments. Anti-cancer compounds used in patients are often targeting prevention of cancer cell proliferation or killing dividing cells. When the compounds are toxic to cancer cells, they may also severely affect normal dividing cells which are necessary for human life. Therefore, one of main directions in cancer studies is to find methods to specifically block or kill cancer cells without affecting normal cell proliferation. There is a demand, now, for such treatment on cancer patients.
ErbBs are class one receptor protein tyrosine kinases. ErbB-mediated cell signalling plays a critical role in embryo development and adult organ function. On a cellular level, ErbB receptors have been shown to mediate signals for cell proliferation, differentiation, migration, and cell structure reorganisation. There are four structurally similar ErbB members, ErbB-1, ErbB-2. ErbB-3 and ErbB-4. The epidermal growth factor (EGF) is one of several ligands that bind ErbB-1. ErbB-3 or ErbB-4 also bind several ligands, including neuregulin-1 (NRG-1). To date, no ligand for ErbB-2 has been identified. However. ErbB-2 serves as an heterodimer partner for ErbB-3, ErbB-4 or ErbB-1, and is critically involved in NRG-1-activated cell signalling.
In vivo studies using gene targeting experiments indicate that developmental defects resulting from inactivation of ErbB-2 are similar to those observed in NRG-1-inactivated animals. Both animals show defects in the neural crania ganglia and heart trabeculae development. Furthermore, ErbB-3 or ErbB-4 gene-inactivated mice have similar or overlapping phenotypes to NRG-1 or ErbB-2 knockout mice.
In addition to its role in development, the human ErbB-2 gene is frequently amplified and its encoded protein is over-expressed in a variety of human carcinomas. Early research on ErbB-2 discovered that an oncogenic point mutation resulted in the formation of ErbB-2 homodimers that in turn caused significant phosphorylation of the tyrosine residues on the intracellular domain. While no corresponding point mutation has been found in ErbB-2 over expressing human carcinomas, the upregulation of ErbB-2 results in the formation of homodimers that in turn increases the tyrosine phosphorylation of its intracellular domain. This process is hypothesised to be the start of a signal cascade that triggers cell transformation and/or growth, and thus initiate tumourigenesis. There is evidence, however, to contradict the hypothesis that ErbB-2 homodimers are responsible for the initiation of tumourigenesis: i) some ErbB-2 mutants that are engineered to enhanced dimerisation and self-phosphorylation have no effect on cell transformation; ii) antibodies that bind to the extracellular domain of ErbB-2 and presumably promote homodimerisation result in ErbB-2-expressing cancer cell growth promotion, whereas others inhibit cancer cell growth. These data indicate that homodimerisation of ErbB-2 is insufficient for cell growth promotion or cell transformation, and other conditions, possibly involving specific dimer orientation or conformation, are required.
ErbB-2 acts as a heterodimer partner for the ligand-binding ErbB-3 or ErbB-4 receptors. The ligand. NRG-1, has been identified to have two independent receptor binding sites: one that has a high affinity for ErbB-3 or ErbB-4, and the other that has a low but non-specific affinity for all ErbB members. Thus, the exposure of NRG-1 to cells expressing ErbB-3/4 and ErbB-2 would result in heterodimers of ErbB-2 and ErbB-3/4. In the absence of the ligand, however, it is unclear whether ErbB-2 has an affinity with other ErbB receptors, and it is possible that such an interaction could be involved in the initiation of cancer. Amongst all the ErbB receptors, ErbB-3 is unique because: i) ErbB-2 preferentially forms heterodimers with ErbB-3; ii) co-transfection of NIH3T3 cells with ErbB-2 and ErbB-3 results in much higher levels of cell transformation than that of transfection with ErbB-2 alone; iii) in ErbB-2 over-expression-associated breast cancer cells. ErbB-3 is also highly expressed: iv) ErbB-3 is also over expressed in ErbB-2-over expressing tumour cells from ErbB-2 transgenic mice.
The present inventors studied the role of ErbBs and their interaction in cell growth and inhibition. Importantly, it was found that homo- and heterodimer formation of ErbBs can play a role in cell proliferation, particularly in cancer cells.